Use PolylineCollection for time-dynamic polylines

In order to keep feature parity between static and time-dynamic polylines, PolylineGeometryUpdater was using PolylineGeometry to render lines that were changing every frame.  Obviously this generated a lot of garbage and would kill performance when using a lot of time-dynamic lines (like the d3 demo does).  In order to improve performance, I've updated the dynamic lines to use PolylineCollection instead; which greatly improves performance.

The only feature PolylineGeometry has that PolylineCollection lacks is per-vertex colors; but since the DataSource layer doesn't expose that feature yet; this is okay for now.  Of course when we have dynamic buffers, this problem should just go away.
This can't be a global.  What if you created two scenes with different CZML loaded?